The present invention relates to flow control arrangements and more particularly to a flow control arrangement utilised within a flow path where depositions may reduce or through-flow or choke a restriction.
Flow control arrangements are provided in which there is a flow restriction—such as an orifice—in a flow path. For example, in order to prevent oil egress from bearing chambers between rotating and static features, it is common practice to provide high pressure sealing air around the periphery of the bearing chamber. The high pressure air then enters the bearing chamber through seals preventing and/or limiting the escape of oil from the sealed mechanism. Once the air flow has passed through the bearing chamber seals and the bearing chamber itself it is understood that the air is taken from that bearing chamber to a low pressure sink. The clearance between the rotating and static seal members is typically specified in order to accommodate the relative movements of the rotor and stator during operation, rather than to control the sealing air flow rate. In such circumstances a flow control function is achieved through utilising a high resistance fitting—typically in the form of an orifice within the vent line connecting the bearing chamber to the low pressure sink.
In order to develop the necessary flow control resistance generally the flow, as indicated, is restricted by an orifice which essentially chokes the flow through a relatively small diameter or cross sectional area for the restrictor. Possibly a restrictor may be in the order of half a millimeter but it is understood that this is dependent upon a particular operational requirements.
FIG. 1 provides a schematic illustration of a typical prior flow control arrangement (1). As can be seen a flow path (2) extends along a flow or vent axis (3) with a flow constriction (4) positioned to extend across the flow path (2) between an upstream side (2a) and a downstream side (2b). Such flow control through the flow restriction may be acceptable where there is pure or uncontaminated air flow but unfortunately flows in accordance with a large number of applications and in particular with regard to seals in a gas turbine engine will generally compromise a mixture of oil droplets carried within the flow in the direction of arrowhead (5). It will also be understood that the flow path (2) may be routed through areas of high temperature or other environmental conditions such that there can be variations in the air to oil mixture composition resulting in localised overheating and/or degradation of the oil droplets with the result that laquering and carbon deposition (6) can occur upon the surfaces of the orifices (4). This carbonisation and laquer deposition can build up over time to a significant thicknesses. The deposits again through potentially thermal cycling and other physical actions including vibration may break up and so form flakes which can then be transported downstream in the flow (5).
FIG. 2 illustrates a potential situation with regard to laquer or deposition break up for the arrangement (1). Thus, as previously explained an air flow which is contaminated with oil droplets passes in the direction of arrowhead (5) within an upstream section (2a) of a flow path. The flow (5) engages a flow restrictor (4) and as previously a deposition has occurred from the oil droplets within the flow (5). As depicted in FIG. 2 a part (6a) of the deposition has become detached. This detached deposition part (6a) may cause a partial or possibly as depicted in FIG. 2 a total blockage of a vent aperture (7) of the restrictor (4). Thus, there will be reduced or no flow through the vent line.
It will be understood that blockage of the flow path will result in cessation of the necessary sealing flows and therefore failure of the sealing mechanism as described above. Similar problems may occur with regard to the functionality achieved by flows with other mechanical arrangements. With regard to a bearing seal, it is understood without the air flow pressurisation there will be leakage of oil from the bearing chamber which may result in potential problems with regard to oil firing, odour and leakage to an external environment as well as degradation of the actual bearing function itself through lack of lubricant. In any event, it will be necessary to clear the aperture (7) by an appropriate remedial action and thus there will be expensive unscheduled maintenance and repair down time for a machine such as a gas turbine engine.